1. Field of the Invention
This invention relates to a high-frequency power divider and, more particularly, to a high-frequency power divider having an isolation resistor at its output side.
2. Description of the Prior Art
In general, a dual-distribution type power divider comprises a flat microwave circuit used as a power divider for distributing or synthesizing high-frequency power. FIGS. 5-(1) and 5-(2) show an example of this circuit.
Referring to FIG. 5-(2), a desired pattern of a power divider circuit is formed on one of two surfaces of a dielectric base 50 in the form of a plate. An earth plate (also known as "ground plate") 52 is attached to the other surface of the dielectric base 50 so that it uniformly covers this surface.
The circuit pattern is formed of a single input-output line 60 which comprises an input stage, a pair of input-output lines 70 and 71 which comprise an output stage, a pair of high-impedance transmission lines 62 and 63 which diverge from the input-output line 60 of the input stage at a diverging point 61 and extends to the input-output lines 70 and 71 of the output stage. An isolation resistor R is interposed between and connected to a pair of output-side portions of the high-impedance transmission lines 62 and 63.
The impedance of each of the input-output line 60 of the input stage and the input-output lines 70 and 71 of the output stage is set to Z [.OMEGA.], and the impedance of each of the high-impedance transmission lines 62 and 63 is set to .sqroot.2.multidot.Z. A length l of each of the high-impedance transmission lines 62 and 63 is determined on the basis of an equation: EQU l=(2n+1).multidot..lambda.g/4
where n=0, 1, 2, . . . , and .lambda.g is the wavelength at the operation frequency.
A reference character 80 denotes an input-output terminal of the input disposed at the input side, and reference characters 81 and 82 denote input-output terminals disposed at the output side.
The example of the conventional device shown in FIG. 5-(2) is represented in an ideal state by an equivalent circuit shown in FIG. 5-(1). In this case, electric power input into the input-output terminal 80 is equally distributed to the input-output terminals 81 and 82. The phase difference between distributed electric powers at input-output terminals 81 and 82 is 0.degree.. Conversely, if electric powers of equal levels are input into the input-output terminals 81 and 82 in phase, an electric power which is synthesized from the input electric powers is obtained at the input-output terminal 80. In this case, since the phases of the two electric powers are equal to each other, no potential difference occurs across the isolation resistor R, and no current flows through the resistor. For this reason, the electric power divider can also serve as an electric power synthesizer.
In the circuit shown in FIG. 5-(1) and, hence, in the ideal state, part of an input supplied through one of the input-output terminals 81 and 82 appears at the input-output terminal 80 while the remaining part is all absorbed by the resistor R so that no power appears at the other one of the input-output terminals 81 and 82.
However, since in the above-described conventional device, the whole of the circuit is provided in the form of a planar microwave circuit on the dielectric base 50, as shown in FIG. 5-(2), each power line for transmitting electric power actually has a uniformly distributed capacitive component. In particular, a predetermined capacitive component C relative to the earth plate 52 is fixed to the reverse surface of the base, attached to the isolation resistor R, as shown in FIG. 5-(3). An impedance mismatch thereby occurs at each of the input-output terminals 80, 81, and 82 so that the input VSWR characteristics deteriorate and reflection waves are increased.